1. Field of the Invention
The present invention relates to a post-start controller for a diesel engine, which reduces rough idle after an engine starts by controlling a heating temperature of a glow plug.
2. Description of the Related Art
In diesel engines, a target fuel injection amount is set with reference to a map based on an engine speed and an engine temperature (typically, a coolant temperature) as parameters in a period from cranking with a starter motor until the engine speed reaches a certain speed. After the engine speed reaches a preset stable speed, the control shifts to post-start control.
In the post-start control, it is checked on the basis of an accelerator opening etc. whether an operating state is an idle operation in which an accelerator pedal is released. If the operating state is the idle operation, a target idle speed is set mainly in accordance with the engine temperature, and feedback control is provided to a fuel injection amount such that a current engine speed becomes the target idle speed.
If individual cylinders have an equivalent combustion state, all cylinders have an equivalent interval rotating speed (an average angular speed from a top dead center to a bottom dead center) in combustion strokes of the individual cylinders, thereby providing a stable engine speed.
In contrast, even when the engine shifts from cold start to idle operation after the engine starts, if the engine temperature is low, combustion becomes unstable, likely producing rough idle. For example, when an interval rotating speed in a combustion stroke of a cylinder is significantly deviated from interval rotating speeds of other cylinders, in a four-cylinder engine in which a combustion stroke is provided every 180 deg-CA (crank angle), for example, waviness of deflection around a crank appears every four combustion strokes. This produces rough idle and causes a driver to feel uncomfortable.
Variation in combustion state among the cylinders is caused by individual differences, such as variation in fuel injection amount from injectors respectively arranged at the individual cylinders, variation in compression ratio among the cylinders, and variation in heating temperature of glow plugs.
For example, Japanese Examined Patent Application Publication No. 6-3168 discloses a technique as a countermeasure to address the above-mentioned problem. The technique detects a rotation variation of an individual cylinder from an engine speed during idle operation, and compares the rotation variation with an average value of rotation variations of all cylinders. If the rotation variation of the cylinder is smaller than the average value, a correction amount for increasing a fuel injection amount of the cylinder is set. If the rotation variation of the cylinder is larger than the average value, a correction amount for decreasing a fuel injection amount of the cylinder is set. Then, to calculate a next fuel injection amount of the cylinder, the fuel injection amount is corrected by using the previously set correction value. Thus, the combustion states among the cylinders are equalized, thereby providing a stable idle rotating speed.
Meanwhile, during idle operation (warm-up operation) after cold start, combustion is unstable. An interval rotating speed in a combustion stroke of a single cylinder is likely significantly deviated from interval rotating speeds in combustion strokes of other cylinders.
The technique disclosed in Japanese Examined Patent Application Publication No. 6-3168 provides the stable idle rotating speed by increasing or decreasing the fuel injection amount of the cylinder having the deviated interval rotating speed. However, if the fuel injection amount of the single cylinder is increased or decreased, an air-fuel ratio may be markedly varied. This may increase emission and, when the fuel injection amount is increased, this may increase fuel consumption.